class ViewPreviewForm

class ViewPreviewForm extends ViewFormBase implements WorkspaceSafeFormInterface {
  
  /**
   * {@inheritdoc}
   */
  public function form(array $form, FormStateInterface $form_state) {
    $view = $this->entity;
    $form['#prefix'] = '<div id="views-preview-wrapper" class="views-preview-wrapper views-admin clearfix">';
    $form['#suffix'] = '</div>';
    $form['#id'] = 'views-ui-preview-form';
    $form_state->disableCache();
    $form['controls']['#attributes'] = [
      'class' => [
        'clearfix',
      ],
    ];
    $form['controls']['title'] = [
      '#prefix' => '<h2 class="view-preview-form__title">',
      '#markup' => $this->t('Preview'),
      '#suffix' => '</h2>',
    ];
    // Add a checkbox controlling whether or not this display auto-previews.
    $form['controls']['live_preview'] = [
      '#type' => 'checkbox',
      '#id' => 'edit-displays-live-preview',
      '#title' => $this->t('Auto preview'),
      '#default_value' => \Drupal::config('views.settings')->get('ui.always_live_preview'),
    ];
    // Add the arguments textfield.
    $form['controls']['view_args'] = [
      '#type' => 'textfield',
      '#title' => $this->t('Preview with contextual filters:'),
      '#description' => $this->t('Separate contextual filter values with a "/". For example, %example.', [
        '%example' => '40/12/10',
      ]),
      '#id' => 'preview-args',
    ];
    $args = [];
    if ($form_state->getValue('view_args', '') !== '') {
      $args = explode('/', $form_state->getValue('view_args'));
    }
    $user_input = $form_state->getUserInput();
    if ($form_state->get('show_preview') || !empty($user_input['js'])) {
      $form['preview'] = [
        '#weight' => 110,
        '#theme_wrappers' => [
          'container',
        ],
        '#attributes' => [
          'id' => 'views-live-preview',
          'class' => [
            'views-live-preview',
          ],
        ],
        'preview' => $view->renderPreview($this->displayID, $args),
      ];
    }
    $uri = $view->toUrl('preview-form');
    $uri->setRouteParameter('display_id', $this->displayID);
    $form['#action'] = $uri->toString();
    return $form;
  }
  
  /**
   * {@inheritdoc}
   */
  protected function actions(array $form, FormStateInterface $form_state) {
    $view = $this->entity;
    return [
      '#attributes' => [
        'id' => 'preview-submit-wrapper',
        'class' => [
          'preview-submit-wrapper',
        ],
      ],
      'button' => [
        '#type' => 'submit',
        '#value' => $this->t('Update preview'),
        '#attributes' => [
          'class' => [
            'arguments-preview',
          ],
        ],
        '#submit' => [
          '::submitPreview',
        ],
        '#id' => 'preview-submit',
        '#ajax' => [
          'url' => Url::fromRoute('entity.view.preview_form', [
            'view' => $view->id(),
            'display_id' => $this->displayID,
          ]),
          'wrapper' => 'views-preview-wrapper',
          'event' => 'click',
          'progress' => [
            'type' => 'fullscreen',
          ],
          'method' => 'replaceWith',
          'disable-refocus' => TRUE,
        ],
      ],
    ];
  }
  
  /**
   * Form submission handler for the Preview button.
   */
  public function submitPreview($form, FormStateInterface $form_state) {
    $form_state->set('show_preview', TRUE);
    $form_state->setRebuild();
  }

}